In these years, surface treated steel strips having high corrosion resistance are required in the fields of automobiles and electric appliances, and development and practical implementation of various zinc system plated steel strips is on the progress. Among others, zinc system hot dipped steel strips including galvanized steel strips (abbreviated as GI, hereinafter) and galvannealed steel strips (abbreviated as GA, hereinafter) now find practical use as rust-preventive steel strips for not only automotive interiors, but also automotive exteriors since they are low in manufacturing cost as compared with zinc electroplated steel strips and have good corrosion resistance. Also in the field of electroplating, alloy coatings obtained by electroplating alloys such as Zn--Ni and Zn--Fe as well as pure zinc can provide steel strips having high corrosion resistance even with relatively low coating weights.
In recent years, to reduce the emission of automobile exhaust gases is recognized as an important problem from global environment considerations and the automobile manufacturers are required to achieve a vehicle weight reduction. Under such circumstances, since a gage down of strip steel is effective for automobile vehicle weight reduction, the material manufacturers are intensely required to supply high tensile strength strip steel, and research and development efforts have been made on high tensile strength steel strips in which Si, Mn, P, Ti, Nb, Al, Ni, Cu, Mo, V, Cr, B, etc. are added to low or ultra-low carbon steel strips as an element for enhancing their strength without detracting from their shapability. Also coupled with the fact that it has been conventionally required to impart rust prevention to steel strips, the automobile manufacturers strongly desire the development of high tensile strength steel strips to which zinc plating, especially zinc hot dipping (galvanizing) featuring a low manufacturing cost is applied.
However, since the above-mentioned reinforcing elements in steel are likely to be oxidized and unlikely to be reduced, a manufacturing line of the Sendzimir type, which is currently a typical continuous manufacturing line of zinc hot dipping, gives rise to an essential problem that these reinforcing elements are selectively oxidized and concentrated at the surface during annealing. In this case, oxides of reinforcing elements such as Si and Mn concentrated at the steel strip surface during annealing substantially detract from the wettability between the steel strip and molten zinc, so that the adhesiveness of molten zinc coating is substantially reduced and in extreme cases, no molten zinc deposits on the steel strip at all, which is known as a non-plating phenomenon. Also, in the case of GA prepared by galvanizing followed by alloying treatment, there accompanies the problem that the alloying temperature must be extremely increased before the alloying treatment can be completed because oxides of reinforcing elements formed during annealing substantially retard alloying.
Also, in the case of zinc electroplating, the above-mentioned in-steel elements are concentrated at the surface to form a tough coating during an annealing step which is prior to the plating step and therefore, the oxide coating resulting from annealing must be mechanically or chemically removed before zinc plating can be electrodeposited on the steel strip.
In order for zinc hot dipping (galvanizing) or zinc electroplating to be applied to such difficultly platable steel strips, there have been developed methods intended to solve the above-mentioned problem by previously carrying out pre-treatment to the steel strips on their surface in order to prevent non-plating.
For example, Japanese Patent Application Kokai (JP-A) Nos. 70268/1982, 79160/1982 and 104163/1983 disclose a method of effecting Fe plating on a steel strip prior to zinc hot dipping (galvanizing).